The recent findings of nitrates in sediment deposits of Gale Crater of Mars, combined with the presence of abundant Fe(II), indicate that nitrate reduction coupled with iron oxidation could be a plausible metabolism supporting microbial life in early Mars environments. Moreover, the identification of organic and inorganic carbon sources and geochemical evidences for the existence of water allow us to assume that nitrate-reducing Fe(II) oxidizing (NRFeOx) microorganisms could have inhabited early Martian fluvial-lacustrine systems active during the Noachian period. In the present study, the Rio Tinto, characterized by high dissolved Fe and seasonal hydrogeochemical fluctuations (as periodical drying, changes in salinity, pH, redox conditions, and evaporation) was considered as a prime environment to detect NRFeOx metabolism analogous to early Mars. Sediment samples from the Rio Tinto were recently collected for geochemical analysis and isolation of NRFeOx microorganisms. High concentrations of nitrate (30 mg/L) as potential electron acceptor and pH values between 4 and 4.5 were observed in the anoxic sediment of the river. Our goal is to enrich and isolate mixotrophic and autotrophic nitrate-reducing Fe(II)-oxidizing microorganisms using the collected sediments with the purpose of addressing environmental microniches of NRFeOx bacteria in Rio Tinto sediments. Moreover, the microbial diversity in the enrichments will be tracked by 16S rRNA (gene) sequencing. We putatively identified denitrifiers (Thiovulaceae, Rhodobacteraceae, Paenibacillaceae, Holophagaceae and and Bacillaceae) as well as Fe(II)-oxidizing Gallionellaceae capable of incomplete denitrification. Furthermore, mixotrophic NRFeOx cultures were already enriched from the anoxic sediments. These preliminary results demonstrated that mixotrophic NRFeOx microorganisms can colonize acidic Fe- and sulfur-rich environments. The potential mineral products of these metabolisms such as Fe(III) (oxyhydr)oxides and Fe(III) hydroxysulfates are promising candidates for biosignature detection especially in the Fe- and sulfate-rich sediments of Gale Crater.

Nitrate-reducing Fe(II)-oxidizing microorganisms in a geochemical and mineralogical Mars terrestrial analogue (Rio Tinto, Spain)

ACS Spring 2022

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